65 



METHODS OF MEASURING SOIL TEMPERATTTRES. 



As it is very iinportant that there should be numerous observations 

 of soil temperature available for af>:ricultural study, and as many 

 persons are deterred by the expensiveness of the deep-earth thermom- 

 eters, I would call attention to the fact that agriculture does not need 

 to consider temperatures at depths below -i feet and that the inex- 

 pensive, excellent system of thermometers, made by Green, of New 

 York, has been recognized as the standard at stations in the United 

 States ; but for accuracy and convenience nothing can exceed the ther- 

 mophone devised by Henry E. Warren and George C. Whipple, of 

 the Massachusetts Institute of Technology. 



Several methods of measuring deep-earth temperatures have been 

 most thoroughly studied in the memoirs of Wild and Leyst, of St. 

 Petersburg, a summary of which I have prepared and will submit 

 at another time. 



The soil thermometers constructed by Green are made in accordance 

 with suggestions made by Milton Wliitney, of the South Carolina 

 Experiment Station, and have been used by him. 



Wliitney has published a description of this new self-registering 

 soil thermometer as follows (see Agr. Sci., Vol. I, p. 253; Vol. Ill, 

 p. 261): 



This is a modification of Six's form of thermometer in which the 

 maximum and minimum temperatures are registered in one and the 

 same instrument. The essential features of the thermometers are as 

 follows: A cylindrical bulb 6 inches long, filled with alcohol. The 

 bulb is protected by a somewhat larger cylindrical metal tube, con- 

 taining numerous holes, and is to be placed 3 inches below the surface 

 of the soil — i. e., so that the bulb will extend vertically between the 

 depths 3 and 9 inches, respectively, in the soil. The tube carrying 

 the alcohol extends some 6 or 8 inches above the surface of the ground, 

 when it bends twice at right angles and descends again to the surface, 

 bends at right angles twice, crossing the main stem, and is carried up 

 aliout G or 8 inches again, where it terminates in a bulb partially filled 

 with alcohol. The lower bend in this stem carries a colunni of mercury 

 Avhich is drawn back toward the bulb when the alcohol contracts, and 

 pushes a steel index up to the minimum temperature on a scale which 

 reads downward. This index is held supported in the alcohol by a 

 little spring Avhen the alcohol expands and the mercury leaves it, 

 while another index is pushed up to the maximum temperature by the 

 other end of the column of mercury. The indices are set by the help 

 of a magnet. 



The advantages claimed for this instrument are that it gives at 

 once, without any calculation, the mean temperature of a definite 

 depth of soil, for which we now use at least three thermometers, 

 while it gives in addition the maximum and minimum temperatures, 

 and need only be read once a day instead of three times, as at 

 present. * * * 

 2667—05 M 5 



